1. Field of the Invention
The present invention relates to a method for producing semiconductor devices suitable as window materials of, for example, photovoltaic cells, photo sensors and the like; and in particular to a method for forming a p-type amorphous semiconductor film for semiconductor devices.
2. Description of the Prior Art
In general, amorphous semiconductors have been produced by chemical vapor deposition (hereinafter referred to as CVD).
In CVD, a gaseous mixture comprising several kinds of source material gases mixed at a desired ratio are flowed into a reaction chamber to be decomposed on a substrate which is in the reaction chamber held under a high vacuum condition and heated to a selected temperature, whereby films are deposited on the substrate.
A photo CVD method using photon energy of ultraviolet rays, a glow discharge plasma CVD method, a thermal CVD method, and the like have been used in the above described generic method for decomposing source material gases.
The photo CVD method is a method in which ultraviolet rays, for example, rays having a wavelength of 184.9 nm, 253.7 nm, etc. generated by a low-pressure mercury lamp are applied to source material gases to decompose them by means of the photon energy of ultraviolet rays.
In the conventional formation of a p-type amorphous silicon carbide film (a-SiC film) by a photo CVD method, disilane (Si.sub.2 H.sub.6) and the like have been used as a main source material gas, diborane (B.sub.2 H.sub.6) as a source of boron is used as a dopant material, and dimethyl silane (SiH.sub.2 (CH.sub.3).sub.2), methyl silane (SiH.sub.3 (CH.sub.3)), acetylene (C.sub.2 H.sub.2) and the like are used as a source of carbon and as a "band gap widening material" for increasing the band gap. This is disclosed in, for example, Appl. Phys. Lett., Vol. 44, No. 9, pp. 871-873 (May 1984) and the like.
With source material gases having the above described composition, diborane, which is used as a source of boron as a dopant material, shows an absorption coefficient for ultraviolet rays lower in comparison with that of disilane, a main source material gas, so that a large amount of diborane is required, as shown in J. Appl. Phys., Vol. 56, No. 10, pp. 2803-2805, (November 1984). Accordingly, much more diborane gas than disilane gas, a main source material gas, is used in the formation of films. However, since diborane, which has not been sufficiently decomposed, remains in this case, the resultant p-type a-SiC film has a reduced conductivity.
In addition, in the case where gases expressed by the general formula SiH.sub.x (CH.sub.3).sub.4-x (methyl silane, dimethyl silane and the like) are used as a source of carbon, their flow rate must be the same as that of the main source material gas.
The above-described method causes a problem, for example, when the flow rate of a mixture gas other than the main source material gas, that is to say, a gas used as a boron source, a gas used as a carbon source and the like, is increased, the flow rate of gases effective for the actual film formation is less than the total flow rate of gases used in the film formation process, whereby the controllability of the film formation reaction is lowered. In addition, if the flow rate of gases other than the main source material gas is increased, the decomposition coefficient of carbon is reduced, whereby undecomposed source material gas is included in the formed film, thereby causing another problem in that film characteristics, such as the optical band gap of the formed p-type a-SiC film, are lowered.
Furthermore, if acetylene is used as a carbon source, the number of C--C bonds in the formed film is increased and consequently the conductivity is badly influenced.
On the other hand, in the glow discharge CVD method in general, monosilane has been used as a source material gas, diborane as a p-type dopant, and methane as a "band gap widening material" for increasing the band gap in the formation of a p-type a-SiC film. This is disclosed in, for example, U.S. Pat. No. 4,385,199 and the like.
However, formation of a p-type a-SiC film by the glow discharge method has caused a problem in that a comparatively large energy is required for the decomposition of both diborane and methane used as source material gases for the glow discharge, the dopant being difficult to introduce into the film, and a resulting p-type a-SiC film having low resistance and a wider optical band gap being difficult to be obtained.